Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data services and multimedia services, beyond providing the early voice-oriented services. Currently, a variety of mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A) of 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) of 3rd Generation Partnership Project 2 (3GPP2), and 802.16 of Institute of Electrical and Electronics Engineers (IEEE), have been developed to support high-speed, high-quality wireless packet data transmission services. In particular, the LTE system that has been developed to efficiently support transmission of high-speed wireless packet data may maximize the capacity of wireless systems by utilizing a variety of radio access technologies. The LTE-A system, an advanced wireless system of the LTE system, has an improved data transmission capability, compared with LTE.
LTE generally means a base station (or a Node B (NB) or an evolved Node B (eNB)) and terminal equipment (or User Equipment (UE), a Mobile Station (MS), a Mobile Equipment (ME), a device or a terminal) corresponding to Release 8 or 9 of the 3GPP standard group, or a communication system or communication technology including them, and LTE-A may mean a base station and terminal equipment corresponding to Release 10 of the 3GPP standard group, or a communication system or communication technology including them. Even after the standardization of the LTE-A system, the 3GPP standard group has been standardizing the next Release that is based on the LTE-A system and has improved performance.
The existing third generation (3G) and fourth generation (4G) wireless packet data communication systems such as HSDPA, HSUPA, HRPD, and LTE/LTE-A may use technologies such as an Adaptive Modulation and Coding (AMC) method and a channel-sensitive scheduling method in order to improve the transmission efficiency.
When utilizing the AMC method, a transmitter may adjust the amount of transmission data depending on the channel state. In other words, if the channel state is not good, the transmitter may reduce the amount of transmission data to match the reception error probability of a desired level, and if the channel state is good, the transmitter may increase the amount of transmission data to effectively transmit a greater amount of information, while matching the reception error probability of the desired level.
If utilizing the channel-sensitive scheduling resource management method, the transmitter may selectively service a user having a good channel state among several users, so that the system capacity may increase, compared with when the transmitter assigns a channel to one user and services the user. This capacity increase is referred to as a so-called multi-user diversity gain.
In short, the AMC method and the channel-sensitive scheduling method are methods in which the transmitter receives partial channel state information from a receiver as feedback information and applies an appropriate modulation and coding technique at the time that is determined to be most effective.
The AMC method, when used with a Multiple Input Multiple Output (MIMO) transmission scheme, may include a function of determining the number of spatial layers (or a rank) for a transmission signal. In this case, in determining the optimal data rate, the AMC method may even consider through how may spatial layers the AMC method will transmit a signal, using MIMO, without simply considering only the coding rate and the modulation scheme.
MIMO for transmitting a wireless signal using a plurality of transmit antennas may be divided into Single User MIMO (SU-MIMO) for transmitting a wireless signal to one terminal and Multi-User MIMO (MU-MIMO) for transmitting a wireless signal to a plurality of terminals using the same time-frequency resources. In the case of SU-MIMO, a plurality of transmit antennas may transmit a wireless signal to one receiver through a plurality of spatial layers. In this case, the receiver should have a plurality of receive antennas to support a plurality of spatial layers. In the case of MU-MIMO, a plurality of transmit antennas may transmit a wireless signal to a plurality of receivers through a plurality of spatial layers.
MU-MIMO, compared with SU-MIMO, has an advantage that the receiver does not require a plurality of receive antennas. However, since the transmitter transmits a wireless signal to a plurality of receivers on the same time-frequency resources, mutual interference may occur between wireless signals for different receivers.
Currently, many studies have been conducted to switch Code Division Multiple Access (CDMA) that is a multiple access scheme used in the second generation (2G) and 3G mobile communication system to Orthogonal Frequency Division Multiple Access (OFDMA) in the next-generation mobile communication system. 3GPP and 3GPP2 have begun to standardize the evolved system that uses OFDMA. It is known that an increase in the capacity may be expected in the OFDMA scheme, compared with in the CDMA scheme. One of the many causes that the OFDMA scheme enables the increase in capacity is that the OFDMA scheme may perform frequency domain scheduling. As the transmitter obtains the capacity gain using the channel-sensitive scheduling method depending on the characteristics that a channel changes over time, the transmitter may obtain more capacity gain if using the characteristics that a channel varies depending on the frequency.
FIG. 1 illustrates time-frequency resources in an LTE/LTE-A system according to the related art.
Referring to FIG. 1, the wireless resources that an eNB transmits to a UE may be divided in units of Resource Block (RB) 100 on the frequency axis, and in units of subframe 105 on the time axis. One RB may include 12 subcarriers and occupy a band of 180 kHz in the LTE/LTE-A system. One subframe may include 14 OFDM symbols and occupy a time period of 1 msec in the LTE/LTE-A system.
In performing scheduling, the LTE/LTE-A system may allocate resources in units of subframe on the time axis, and in units of RB on the frequency axis.
FIG. 2 illustrates a wireless resource of one subframe and one RB, which is a minimum unit that may be scheduled as a downlink in an LTE/LTE-A system according to the related art.
Referring to FIG. 2, a wireless resource may include one subframe on the time axis and one RB on the frequency axis. This wireless resource may include 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, so the wireless resource may have a total of 168 unique time-frequency positions. In LTE/LTE-A, each unique time-frequency position in FIG. 2 will be referred to as a Resource Element (RE). In addition, one subframe may include two slots each having 7 OFDM symbols.
A plurality of different types of signals may be transmitted on the wireless resource shown in FIG. 2. The different types of signals may include a Cell Specific Reference Signal (CRS) 200, a Demodulation Reference Signal (DMRS) 202, a Physical Downlink Shared Channel (PDSCH) 204, a Channel Status Information Reference Signal (CSI-RS) 206 or other control channel 208.
CRS is a reference signal that is transmitted for all UEs belonging to one cell (i.e., a cell-specific signal).
DMRS is a reference signal that is transmitted for a specific UE (i.e., a UE-specific signal).
A PDSCH signal is a signal of a data channel that is transmitted on a downlink. The PDSCH signal is used by an eNB to transmit traffic to a UE, and is transmitted using an RE(s) where no reference signal is transmitted in a data rage 210 of the wireless resource.
CSI-RS is a reference signal that is transmitted for multiple user equipment (UEs) belonging to one cell, and is used to measure a channel state. A plurality of CSI-RSs may be transmitted to one cell.
The other control channel signal 208 may be a signal for providing control information that a UE requires in receiving a PDSCH, or an Acknowledgment/Negative-Acknowledgment (ACK/NACK) signal for operating Hybrid Automatic Repeat reQuest (HARQ) for data transmission of an uplink. For example, the control information may include a Physical Hybrid-ARQ Indicator Channel (PHICH), a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH) and/or the like.
In addition to the above signals, the LTE-A system may set muting so that CSI-RS transmitted by another eNB may be received at UEs in the cell without interference. The muting may be applied in the position where CSI-RS may be transmitted. In this case, a UE may generally receive a traffic signal by skipping the wireless resource. In the LTE-A system, muting may be referred to as another term ‘zero-power CSI-RS’, because muting is applied to the position of CSI-RS and no transmission power is transmitted.
Referring to FIG. 2, CSI-RS may be transmitted using some of the positions indicated by A, B, C, D, E, E, F, G, H, I and J depending on the number of antennas for transmitting CSI-RS. In addition, the muting may also be applied to some of the positions indicated by A, B, C, D, E, E, F, G, H, I and J.
In particular, CSI-RS may be transmitted on 2, 4 or 8 REs depending on the number of transmit antenna ports. If the number of antenna ports is 2, CSI-RS may be transmitted on a half of a specific pattern in FIG. 2, and if the number of antenna ports is 4, CSI-RS may be transmitted on the whole of a specific pattern. If the number of antenna ports is 8, CSI-RS may be transmitted using two patterns.
On the other hand, muting may be done in units of one pattern at all times. In other words, muting may be applied to a plurality of patterns, but if muting does not overlap CSI-RS in terms of the position, muting may not be applied only to one pattern. However, only if CSI-RS overlaps muting in terms of the position, muting may be applied only to a part of one pattern.
In a cellular system, in order to measure a downlink channel state, an eNB should transmit a reference signal. In the case of the LTE-A system, a UE may measure a channel state between the eNB and the UE, using CRS or CSI-RS transmitted by the eNB.
For the channel state, several factors should be considered basically, and the factors may include the amount of interference in a downlink. The amount of interference in a downlink may include an interference signal and a thermal noise generated by an antenna belonging to a neighboring eNB, and it is important for a UE to determine the channel status of the downlink. For example, if an eNB with one transmit antenna transmits a signal to a UE with one receive antenna, the UE should determine a Signal to Noise plus Interference Ratio (SNIR) by determining energy per symbol that the UE may receive on a downlink in the reference signal received from the eNB, and the amount of interference that is to be received at the same time in the period where the UE receives the symbol. The SNIR is a value determined by dividing the power of a received signal by the strength of a noise signal. Generally, as SNIR is higher, the UE may obtain the higher reception performance and the higher data transfer rate. The determined SNIR, the value corresponding to the SNIR or the maximum data transfer rate supportable at the SNIR may be notified to the eNB, allowing the eNB to determine at which data transfer rate it will perform downlink transmission to the UE.
In the case of the general mobile communication system, eNB equipment may be deployed at the mid-point of each cell, and the eNB equipment may perform mobile communication with the UE using one or a plurality of antennas disposed in a limited place. The mobile communication system in which antennas belonging to one cell are disposed in the same position is referred to as a Centralized Antenna System (CAS). On the other hand, the mobile communication system in which antennas (or Remote Radio Heads (RRHs)) belonging to one cell are disposed in distributed positions within the cell is referred to as a Distributed Antenna System (DAS).
FIG. 3 illustrates arrangement of antennas in distributed positions in a general distributed antenna system according to the related art.
Referring to FIG. 3, a distributed antenna system with two cells 300 and 310 is illustrated.
For example, the cell 300 may include one high-power antenna 320 and four low-power antennas (e.g., an antenna 340). The high-power antenna 320 may provide a minimum service over the entire area within the cell coverage, and the low-power antenna 340 may provide a service that is based on the high data rate, to the limited UEs in a limited area within the cell. In addition, the low-power antenna 340 and the high-power antenna 320 may both be connected to a central controller (see 330), and operate depending on the scheduling and the wireless resource allocation by the central controller. In the distributed antenna system, one or a plurality of antennas may be disposed in one geographically separated antenna position. In the distributed antenna system, an antenna or antennas disposed in the same position will be referred to as an antenna group (or RRH group) in the present disclosure.
In the distributed antenna system shown in FIG. 3, a UE may receive a signal from one geographically separated antenna group, whereas a signal transmitted from another antenna group may act as interference to the UE.
FIG. 4 illustrates an interference phenomenon that occurs when each antenna group performs transmission to different UEs in a distributed antenna system according to the related art.
Referring to FIG. 4, solid arrows represent desired (or valid) signals, and dashed arrows represent interference signals. A UE1 400 is receiving a traffic signal from an antenna group 410. However, a UE2 420 is receiving a traffic signal from an antenna group 430, a UE3 440 is receiving a traffic signal from an antenna group 450, and a UE4 460 is receiving a traffic signal from an antenna group 470. While receiving a traffic signal from the antenna group 410, the UE1 400 may receive interference signals from the other antenna groups that are transmitting traffic signals to other UEs. In other words, the signals transmitted from the antenna groups 430, 450 and 470 may generate an interference effect to the UE1 400.
In the distributed antenna system, interference generated by different antenna groups may include two types of interference (i.e., inter-cell interference and intra-cell interference). The inter-cell interference refers to interference that is generated between antenna groups of different cells, and the intra-cell interference refers to interference that is generated between (different) antenna groups of the same cell.
The intra-cell interference that the UE1 400 in FIG. 4 experiences may include interference that is generated in the antenna group 430 belonging to the same cell (i.e., a cell 1), and the inter-cell interference that the UE1 400 experiences may include interference that is generated in the antenna groups 450 and 470 of the adjacent cell (i.e., a cell 2). The inter-cell interference and the intra-cell interference may be received at the UE at the same time, interfering with reception of a data channel by the UE.
Generally, if a UE receives a wireless signal, the desired signal may be received together with noise and interference. In other words, the received signal may be expressed by the following Equation 1.r=s+noise+interfernce  Equation 1where ‘r’ denotes a received signal, ‘s’ denotes a transmission signal, ‘noise’ denotes a noise having a Gaussian distribution, and ‘interference’ denotes an interference signal generated in wireless communication.
The interference signal may be generated even in an adjacent transmission point (e.g., an adjacent cell), and even in the same transmission point (e.g., a serving cell). The interference in the adjacent transmission point refers to a case where the signal that is transmitted by an adjacent cell or an adjacent antenna in the distributed antenna system acts as interference to the desired signal. The interference in the same transmission point refers to a case where when MU-MIMO transmission is performed in one transmission point using a plurality of antennas, signals for different users cause mutual interference.
A value of SNIR varies depending on the strength of interference, resulting in an influence on the reception performance. Generally, the interference may be the most significant factor that may hinder the system performance in the cellular mobile communication system, and how to appropriately control the interference may determine the system performance.
In LTE/LTE-A, introduction of a variety of standard technologies for supporting Network Assisted Interference Cancellation and Suppression (NAICS) technology as a method capable of increasing the reception performance in the situation where interference occurs has been taken into consideration. The NAICS technology is technology in which an eNB delivers information related to an interference signal to a UE over the network and the UE recovers the received signal in consideration of the characteristics of the interference signal using the received information. For example, if the UE is aware of the modulation scheme and the reception strength for the interference signal, the UE may cancel the interference signal or recover the received signal in consideration of the interference signal, thereby improving the reception performance.
In the wireless communication system, error correction coding may be performed in order to correct errors in a transmission/reception process. In the LTE/LTE-A system, a convolutional code, a turbo code and/or the like may be used for error correction coding.
In order to increase the decoding performance of the error correction coding, a receiver may use soft decision instead of hard decision when demodulating modulation symbols that are modulated by Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM) and 64QAM. If a transmitter transmits ‘+1’ or ‘−1’, a receiver employing hard decision may select one of ‘+1’ or ‘−1’ for a received signal and output the selected one. On the other hand, a receiver employing soft decision may output information about any selected one of ‘+1’ or ‘−1’ for a received signal, and a reliability of the decision, together. The reliability information may be utilized in improving the decoding performance in a decoding process.
What the receiver employing soft decision generally uses in calculating its output value may be a Log Likelihood Ratio (LLR). If the Binary Phase Shift Keying (BPSK) modulation scheme, in which the transmission signal is one of ‘+1’ or ‘−1’, is applied, LLR may be defined as follows.
                    LLR        =                  log          ⁢                                          ⁢                                    f              ⁡                              (                                                      r                    |                    s                                    =                                      +                    1                                                  )                                                    f              ⁡                              (                                                      r                    |                    s                                    =                                      -                    1                                                  )                                                                        Equation        ⁢                                  ⁢        2            where ‘r’ denotes a received signal and ‘s’ denotes a transmission signal. In addition, a conditional probability density function ƒ(r|s=+1) is a probability density function of a received signal under the condition that ‘+1’ is transmitted as a transmission signal. Similarly, a conditional probability density function ƒ(r|s=−1) is a probability density function of a received signal under the condition that ‘−1’ is transmitted as a transmission signal. Even for the modulation scheme such as QPSK, 16QAM and 64QAM, LLR may be expressed by an equation in a similar manner. The conditional probability density function has a Gaussian distribution in the situation where no interference exits.
FIG. 5 illustrates an example of a conditional probability density function according to the related art.
Referring to FIG. 5, a curve indicated by reference numeral 500 corresponds to a conditional probability density function ƒ(r|s=−1), and a curve indicated by reference numeral 510 corresponds to a conditional probability density function ƒ(r|s=+1) Using the conditional probability density function, if a received signal value is the same as a point indicated by reference numeral 520, a receiver may calculate LLR as log (f2/f1). The conditional probability density function in FIG. 5 may be assumed to be a case where the noise and interference follow a Gaussian distribution.
In the mobile communication system such as LTE/LTE-A, an eNB may deliver information of tens of bits to a UE with one transmission of PDSCH. The eNB may encode information to be transmitted to a UE, modulate the coded information by a modulation scheme such as QPSK, 16QAM and 64QAM, and transmit the modulated information. Therefore, a UE that has received PDSCH may generate LLRs for tens of coded symbols in a process of demodulating tens of modulation symbols, and deliver the generated LLRs to a decoder.
While noise follows a Gaussian distribution, interference may not follow the Gaussian distribution. The typical reason why the interference does not follow the Gaussian distribution is because the interference, unlike the noise, is a wireless signal for another receiver. In Equation 1, since ‘interference’ is a wireless signal for another receiver, the interference may be transmitted after the modulation scheme such as BPSK, QPSK, 16QAM and 64QAM is applied thereto. For example, if an interference signal is modulated by BPSK, the interference may have a probability distribution having a value of ‘+k’ or ‘−k’ as the same probability, where ‘k’ is a value determined by the signal strength attenuation effect of a wireless channel.
FIG. 6 illustrates a conditional probability density function in a case where it is assumed that an interference signal is also transmitted by a BPSK modulation scheme in the situation where a received signal is transmitted by the BPSK modulation scheme according to the related art.
Referring to FIG. 6, noise is assumed to follow the Gaussian distribution.
The conditional probability density function in FIG. 6 may observe a different one from that of the conditional probability density function in FIG. 5. Referring to FIG. 6, a curve indicated by reference numeral 620 corresponds to a conditional probability density function ƒ(r|s=−1), and a curve indicated by reference numeral 630 corresponds to a conditional probability density function ƒ(r|s=+1).
In addition, a length of an interval indicated by reference numeral 610 is determined depending on the signal strength of the interference signal, and may be determined depending on the influence of a wireless channel. Using the conditional probability density function, if a received signal value is the same as a point indicated by reference numeral 600, a receiver may calculate LLR as log (f4/f3). This value may have a different value from the LLR value in FIG. 5, because the conditional probability density function is different. In other words, the LLR determined by considering the modulation scheme of an interference signal is different from the LLR that is calculated assuming the Gaussian distribution.
FIG. 7 illustrates a conditional probability density function in a case where it is assumed that an interference signal is transmitted by a 16QAM modulation scheme in a situation where a received signal is transmitted by a BPSK modulation scheme according to the related art.
Referring to FIG. 7, a curve indicated by reference numeral 700 corresponds to a conditional probability density function ƒ(r|s=−1), and a curve indicated by reference numeral 710 corresponds to a conditional probability density function ƒ(r|s=+1).
FIG. 7 shows that as the modulation scheme of an interference signal is different from that of a received signal, the conditional probability density function may be different. While a received signal is transmitted by the BPSK modulation scheme in both FIGS. 6 and 7, FIG. 6 corresponds to a case where an interference signal is transmitted by the BPSK modulation scheme, and FIG. 7 corresponds to a case where an interference signal is transmitted by the 16QAM modulation scheme. In other words, it may be seen that even though the modulation scheme of a received signal is the same, the conditional probability density function may be different depending on the modulation scheme of an interference signal, and as a result, the calculated LLR may also be different.
Referring to FIGS. 5, 6, and 7, LLR may have a different value depending on how the receiver assumes interference to calculate the LLR.
In order to optimize the reception performance, it is necessary to calculate the LLR using a conditional probability density function determined by reflecting the statistical characteristics of the actual interference, or to calculate the LLR after cancelling the interference in advance. In other words, if interference is transmitted by the BPSK modulation scheme, the receiver should calculate the LLR assuming that the interference is transmitted by the BPSK modulation scheme, or calculate the LLR after cancelling the interference modulated by BPSK. If interference is transmitted by the BPSK modulation scheme, the receiver may calculate the LLR value that is not optimized, when the receiver, without performing the interference cancellation procedure, simply assumes that the interference has a Gaussian distribution or assumes that interference is transmitted by the 16QAM modulation scheme. As a result, the receiver may fail to optimize the reception performance.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.